Magnetron



April 8,' 1952 R. L. sPRouLL 2592206 MAGNETRON Filed March 16, 1946 2 SI-lEETSfSHEET l A'ITORN pril 8, 1952 R, SPRQULL 2,592,206

MAGNETRON Filed Marh 16, l1948 2 sHEE'rs-SHEET 2 INVENTOR 'Oy/W14 ATToRN '4l Patented Apr. 8, 1952 MAGNETRON Robert L. Sproull, Penns Neck, N. J., assignor to Radio Corporation of America,'a corporation of Delaware Application March 16, 1946, Serial No. 654,934

23 Claims.

My invention relates to electron discharge devices useful at very high frequencies and more particularly to magnetrons.

In a magnetron oscillator some of the electrons, of the order of one-half of the total, are emitted from the cathode in incorrect phase and instead of giving energy to the radio frequency field, absorb energy from the field and return to the cathode. To date no simple method of preventing this back bombardment has been proposed. This back bombardment has three disadvantages, namely: (l) dissipation by the cathode of some of the input power, of the order of 5%' of the input power; (2) reduction in cathode life. The bombarding electrons remove the cathode coatingand the life of the magnetron is therefore much less thanthat of ordinary tubes. (3) The removed material may be ionized in the interaction space between the cathode and anode. This space ionization produces large noise modulation of the output power under some conditions. The first of these defects reduces the eiciency somewhat, but i's nota serious defect. The other two eifects are most undesirable.

It is,therefore, a principal object of my invention to provide an improved form of electron discharge device particularly of the magnetron type.

A further object of my invention is to provide a magnetron useful as an oscillator and having a longer life than those magnetrons now available.

A still further object of my invention is to provide an electron discharge device of the magnetron type in which the large noise modulation of the output power is substantially eliminated.

More specifically t is an object of my invention to neutrali'ze the effect of returned electrons in a magnetron.

In accordance with my invention, I achieve the foregoing objects by providing an elongated generally cylindrical cathode having longitudinally-extending major surfaces disposed around the per'iphery thereof, with alternate surfaces only being coated with electron emitting material, and with said sur'faces extending generally outwardly from the cathode proper and the interme'diate uncoated surfaces serving as shields for' the coated surfaces, to prevent back bombardment of the latter by returning electrons.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 is a transverse schematic section of an electron discharge device illustrating the problem solved by my invention, Figure 2 is a. transverse section taken along the line 2-2 of Figure 3 and showing details of construction of an electron discharge device made according to my invention, Figure 3 is a longitudinal section taken along the line 3-3 of Figure 2, and Figures 4 and' 5 are transverse sections of'modifications of the cathode construction utilized in the tubes shown in Fgures 2 and 3.

As pointed out above, the dissipation at the cathode of some of the input power is not serious. The other two effects considered, however, are avoided or substantially eliminated by my invention.

Referring to Figure 1, I- show a transverse section of a conventional magnetron employing an indirectly heated cathode 9 which may be an elongated tubular member provided with heater, not shown, mounted within an anode block lu provided with a plurality of radially directed slat-like members ll, the inner ends of which provide anode elements or segments 12 defining the anode-cathode space. The slat-like` members H with the back of the anode block Ill provide cavity resonators between adjacent anode elements |2. The path 'l represents the orbit of an electron leaving the cathode in the wrong phase and causing back bombardment of the cathode. The path 8 represents the orbit of an electron which reaches the anode. These orbits have been established in a general way by those who have worked on the theory of magnetron operation. It will be noted that the electron following path 'l strikes the cathode at an angle of approximately 10 to 45 and not normal thereto.

My invention provides a cathode which will eliminate the deleterious effects caused by electrons such as those following path 1 in knockng the emitting material from the surface of the cathode and thus causing a shortening of the life of the cathode and ionization in the interaction space which produces the large noise modulation of the output power under some conditions.

Referring to Figure 2, the anode block Ill is provided With a plurality of inwardly radially directed members Il, the inner ends of which define a space in which the indirectly heated tubular cathode |3 having the heater HI is positioned. The inner ends of the members H provide the anode elements |2 connected by cavity resonators as described above. The anode block IO is closed by the end cover members l'l and |8 and the cathode is supported by means of the glass bead and wire support members and IG from the radially directed members I, as shown in Fig. 3. The cathode heater leads 19 and 20 are insulatingly sealed at |9' and 20' through one of the cover members |8. The whole assembly is hermetically sealed to provide a vacuum-tight envelope. The magnetic field is provided by means of the magnet poles 2| and 22, for example, of a permanent magnet. Energy is coupled out of the resonator in a well-known manner by means of a coaxial line having an outer tubular conductor 23 and inner conductor 24 terminating in the coupling loop 25.

In accordance with my invention the indirectly heated cathode l3 comprises a hollow tubular element of generally-circular cross-section with outwardly-extending longitudinal ribs 21 producing a star-shaped transverse section and in the particular tube shown the cathode was machined of nickel. As indicated, each of the ribs 21 of the star is coated with electron emitting material on one side face 28 only and left uncoated on the other (face 29. By providing proper direction of the magnetic field so that electrons leaving the coated surfaces 28 travel in circular -paths in the direction indicated by the arrow 40, the returning electrons strike the uncoated nickel surfaces 29. Thus the solid material of the ribs 21 serves as shielding means for the adjacent coated surfaces 28 on the ribs, so that back bombarding electrons cannot strike the coated surfaces 28, and hence, cannot remove the cathode coating material. The result is longer life of the cathode and hence the useful life of the tube is thereby extended and excess noise modulation by gas ionization is prevented. As shown in Fig. 2, the surfaces of the ribs 21 are inclined at an angle of approximately 60 with respect to a tangent to a cylinder circumscribed about the cathode, and hence, returning electrons which approach the cathode at angles less than 60 cannot bombard the coated faces 28.

In Figure 4 I show a modification of the cathode shown in Figures 2 and 3. Here the tubular cathode 30 is again provided with the cathode heater 30' and has secured to its outer surface longitudinally extending radially directed ribs or fins 3| coated on one side face 32 and uncoated on the other side face 33. Operation is as indicated in Figures 2 and 3. It can be seen that the radial arrangement of Fig. 4 prevents the coated faces 32 from being bombarded by returning electrons approaching the cathode at angles less than 90.

In Figure 5 I show a still further modification of my invention. In this form the elongated tubular cathode sleeve 35 has punched out from it ribs or fins 36 coated on one side face 31 and uncoated on the other side face 38. This arrangement protects the coated faces 38 at angles up to approximately 45. Thus in all arrangements shown the surfaces of the cathode are so formed that the coated surfaces are not exposed to bombardment by returned electrons. rThis is accomplished by providing outwardly-extending longitudinal ribs on the cathode. As a result the life of the tube is increased and excess noise is decreased.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application 'for which my invention may be employed,

it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular'structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. An electron discharge device having a plurality of anode elements defining a space, means including a cathode positioned within said space to supply electrons therein and means adjacent said anode elements for generating a magnetic field within said space parallel to the longitudinal axis of said cathode whereby electrons from said cathode are made to travel curved paths during operation of said electron discharge device, said cathode having a plurality of coated emitting surfaces and uncoated surfaces thereon, the space between said cathode and said anode elements being unobstructed, said coated surfaces being disposed out of the paths of electrons leaving the cathode surface and returning to the cathode surface along curved paths.

2. An electron discharge device having a plurality of anode elements defining a space, means including a cathode positioned within said space to supply electrons therein and means adjacent said anode elements for generating a magnetic field within said space parallel to the longitudinal axis of said cathode whereby electrons from said cathode are made to travel curved paths during the operation of said electron discharge device, said cathode having elongated flat surfaces extending longitudinally of said cathode, alternate ones only of said surfaces having emitting coatings, said alternate surfaces being inaccessible to electrons returning to said cathode along curved paths.

3. An electron discharge device having a plurality of anode elements defining a space, means including a cathode positioned within said space to supply electrons therein and means adjacent said anode elements for generating a magnetic field within said space parallel to the longitudinal axis of said cathode whereby electrons from said cathode are made to travel curved paths during the operation of said electron discharge device, said cathode having surfaces lying along planes passing radially through the longitudinal axis of said cathode, alternate surfaces only being coated, said coated surfaces being inaccessible to electrons returned to said cathode along curved paths.

4. An electron discharge device having a plurality of anode elements defining a space, means including a cathode positioned within said space to supply electrons therein and means adjacent said anode elements for generating a magnetic field within said space parallel to the longitudinal axis of said cathode Whereby electrons from said cathode are made to travel curved paths during the operation of said electron discharge device, said cathode including a hollow tubular member having fins extending from the outer surface thereof, said fins being coated with emitting material on one side only, said coated surfaces being inaccessible to electrons returned to said cathode along curved paths.

5. An electron discharge device having a plurality of anode elements defining a space, means including a cathode positioned within said space to supply electrons therein and means adjacent said anode elements for generating a magnetic field within said space parallel to the longitudinal axis of said cathode Whereby electrons from said cathode are made to travel curved paths during the operation of said electron discharge device, said cathode having a star-shaped transverse section whereby longitudinally extending surfaces lie at an angle to each other, and emitting coatings on alternate surfaces only, said alternate surfaces being inaccessible to electrons returned to said cathode along said curved paths.

6. An electron discharge device having a plurality of anode elements defining a space, means including a cathode positioned Within said space to .supply electrons therein, means adjacent said anode elements for generating a magnetic field within said space parallel to the longitudinal axis of said cathode whereby electrons from said cathode are made to travel curved paths during the operation of said electron discharge device, said cathode having flat surfaces extending longitudinally of said cathode, said surfaces lying in planes at angles to each other, alternate surfaces only being coated, said alternate surfaces being inaccessible to electrons returning to said cathode along curved paths.

7. An electron discharge device having a -plurality of anode elements defining a space, means including a cathode positioned within said space to supply electrons therein, and means adjacent said anode elements for generating a magnetic field within said space parallel to the longitudinal axis of said cathode whereby electrons from said cathode are made to travel curved paths during the operation of said electron discharge device, said cathode including a hollow tubular member having punched-out fins extending outwardly from the outer surface thereof, and said fins having coated surfaces on one side only and inaccessible to electrons returned to said cathode along curved paths.

8. An electron discharge device having a plurality of anode elements defining a space, means including a cathode positioned within said space to supply electrons therein, and means adjacent said anode elements for generating a magnetic field Within said space parallel to the longitudinal axis of said cathode Whereby electrons from said cathode are made to travel curved paths during operation of said electron discharge device, said cathode having a plurality of outwardly-projecting longitudinal ribs with emitting coatings on one side surface only, said emitting coatings being inaccessible to electrons returning to said cathode along curved paths.

9. An electron discharge device having a plurality of elongated parallel anode elements defining a cylindrical space, means including an elongated cathode positioned Within said space with its longitudinal axis parallel to said anode elements to supply electrons within said space, and means adjacent said anode elements for generating a magnetic field Within said space vparallel to the longitudinal axis of said cathode whereby electrons from said cathode are made to travel curved paths during operation of said space, means including a lcathode of generally circular cross-section positioned Within said space to supply electrons therein, and means adjacent said anode elements for generating a magnetic field Within said space parallel to the longitudinal axis of said cathode Whereby electrons from said cathode are made to travel curved electron discharge device, said cathode comprising an elongated tubular element having a pluralityv of outwardly-projecting longitudinal ribs uniformly spaced 'around the periphery thereof and having emitting coatings on alternate side surfaces only, said emitting coatings being inaccessible to electrons returning to said cathode along curved paths.

10. An electron discharge device comprising a plurality of anode elements defining a cathode` paths during operation of said device, said cathode including a plurality of surfaces disposed around the periphery thereof, alternate ones of said surfaces being coated with emitting material and the remaining surfaces being uncoated, said coated-surfaces being disposed out of the paths of electrons leaving said cathode and returning to the cathode along curved paths during operation of said device.

11. An electron discharge device comprising an electron-emissive cathode, an anode adjacent but spaeed from said cathode, and means adjacent said lcathode and anode for generating a magnetic field substantially normal to the paths of electrons from said cathode toward said anode, whereby electrons are caused to return to said cathode along curved paths during operation of said device, said cathode including a plurality of outwardly-projecting ribs, alternate side surfaces only of said ribs being coated with emissive material and being inaccessible to said electrons returning to said cathode.

12. A cathode comprising an elongated tubular element having a plurality of outwardly-projecting longitudinal ribs uniformly disposed around the periphery thereof and having emitting coatings on alternate side surfaces only of said ribs.

13. An elongated cathode comprising an element of generally circular cross section having a plurality of discrete, longitudinallyr-extending, major surfaces of substantially equal area disposed around the periphery thereof, alternate ones only of said major surfaces being coated with emitting material and the intermediate major surfaces being uncoated, each of said coated major surfaces extending generally outwardly from said element.

14. An elongated cathode of generally circular cross section having surfaces lying along planes passing radially through the'longitudinal axis thereof, alternate surfaces only being coated with emitting material.

15. A cathode including a hollow tubular member having longitudinal fins extending from the outer surface thereof, said fins being coated with emitting material on alternate side faces only.

16. An elongated cathode having a star-shaped transverse section With adjacent longitudinallyextending surfaces lying at an angle to each other, and emitting coatings on alternate surfaces only.

17. An elongated cathode comprising an element of generally circular cross section having a plurality of discrete, outwardly-projecting, longitudinally-extending, fiat major faces disposed around the periphery thereof, alternate ones of said major faces being coated with emitting material and the intermediate major faces being uncoated.

18. A cathode according to claim 17 wherein adjacent ones of said major faces are disposed in ntersecting planes.

19. A cathode according to claim 17, wherein said major faces are disposed in pairs, with the faces of each pair in parallel planes and facing in opposite directions.

20. An electron discharge device comprising a generally cylindrical cathode, a hollow anode coaxially surrounding said cathode, said cathocle comprising a plurality of surfaces coated with electron emissive material, means for generating a magnetic field substantially parallel to the longitudinal axis of said cathode and anode, whereby part of the electrons emitted by said cathode toward said anode are caused to return toward said cathode along curved paths during operation of said device, and uncoated shielding means adjacent to said surfaces and disposed out of the paths of at least half of the electrons leaving said surfaces towai'd said anode but in the paths of said electrons returning toward said cathode along said curved paths and approaching said cathode at angles at least up to approximately 45 thereto, to prevent back bombardment of said coated surfaces by said returning electrons.

21. An electron discharge device according to claim 20, wherein said shielding means is located in the paths of electrons approaching said cathode at angles up to approximately 60 thereto.

22. An electron discharge device according to claim 20, wherein said shielding means is dis- 8 posed in the paths of electrons approaching said cathode at angles up to approximately 90 thereto. 23. An elongated cathode having a plurality of outwardly-projecting longitudinal ribs each having two exterior side faces, alternate side faces only being coated With emitting material.

ROBERT L. SPROULL.

REFEREN CES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'ENTS 

